With rapid growth of offshore drilling and constant development of horizontal well technology, a logging while drilling (LWD) technology has been increasingly used in a widespread manner. The LWD technology is distinguished from conventional wireline logging substantially by real-time data acquisition. That is, formation data can be acquired without invasion or with merely slight invasion of a drilling fluid, and therefore can reflect the conditions of undisturbed zone more accurately. Formation data are tested and transmitted to the ground for on-site analysis and interpretation during well drilling. This not only shortens a drilling cycle, but also provides guidance of well drilling, adjustment of drilling trajectories, and improvement of drilling procedures. Therefore, how to manage signal transmission from a bottom hole to the ground is both an essential step in the LWD technology and one of the bottlenecks restricting development thereof.
Currently, a real-time transmission mode and a storage transmission mode are used to achieve signal transmission from the bottom hole to the ground. According to the real-time transmission mode, various wired or wireless transmission approaches can be employed to transmit measurement while drilling (MWD) data to the ground in time. The real-time transmission mode is of paramount importance to guidance of well drilling, especially to geosteering during well drilling. At present, however, there are hardly any data transmission approaches that can satisfy the requirements for prompt and effective transmission of large amounts of data from the bottom hole to the ground. The storage transmission mode means that LWD data are directly stored in a measuring tool, and then read out with a cable when an MWD instrument is lifted to the ground at trip out. This mode, although can be used to accomplish collection of large amounts of data, cannot meet the real-time requirement.
Wired transmission approaches include cable transmission, optical fiber transmission, and drill shaft transmission ones. Document 1 (“Researches on Intelligent Drillstring Information and Power Transmission System,” Petroleum Drilling Techniques, 2006, 34(5), pp 10-13) discloses an approach of signal transmission through cables while drilling, comprising putting an armored cable down into a drill shaft, followed by signal transmission. However, as the drilling depth increases, the cable and an MWD instrument have to be lifted to the ground when an additional cable is necessary, or alternatively, such an additional cable has to be inserted into inner bores of the drill shaft in advance. Document 2 (“New Technology of MWD Data Transmission,” Petroleum Instruments, 2004, 18(6), pp 26-31) discloses an approach of optical fiber transmission, comprising putting an optical fiber having a protective layer down into a well, and connecting the optical fiber to the ground via an MWD instrument placed at the bottom hole, such that MWD data can be transmitted via the optical fiber. Since the optical fiber and the cable have the same functions, they bring about the same problems also. Document 3 (“Status Quo and Prospects of Rotary Steerable Drilling Technology,” China Petroleum Machinery, 2006, 34(4), pp 66-70) discloses an approach of drill shaft transmission, comprising mounting a conductor into a drill shaft, and allowing the conductor to become a part of an integral drill shaft, wherein a special connection module mounted on a drill shaft joint enables an entire drill string to form an electrical signal passage, thus achieving data transmission.
The above approaches, due to adoption of wired connection, have the advantage of rather fast transmission rates, much faster than those of wireless approaches. However, the cable, the optical fiber, and the special drill shaft connector have to be mounted to a whole wellbore. During well drilling, the drill shaft rotating at a speed will render these wired media easily damaged. As can be seen, these prior arts have the same defects as inferior reliability, relatively complex manufacturing procedures, and frequent interference with normal drilling procedures. As a result, the above techniques are not quite commonly used in practical manufacturing procedures through the LWD technology.
Wireless transmission approaches include use of mud (drilling fluid) pulse, electromagnetic wave, and acoustic wave approaches, among which the mud pulse and electromagnetic wave approaches have been used in practical LWD production, and the mud pulse approach is most widely used. Chinese patent application 201020298582.3, entitled “High-speed transmission sending device for measurement while drilling,” discloses a mud pulse signal generator, mainly comprising a discharge valve or a throttle valve, wherein when the valve is in an open or closed state, variation of flow rates of a drilling fluid flowing to an annular space in a drill string will cause drilling fluid pressure waves in the drill shaft to generate a series of pulses, and data can thus be transmitted to the ground by being loaded to these pulses via opening and closing of the valve. However, mud waves, being mechanical waves, have largely restricted speeds due to a modulation mode thereof. The highest transmission speed that has been reported so far reaches merely dozens of bits of data per second, which can hardly satisfy the requirements for fast transmission of data from the bottom hole to the ground. CN 102251769A, entitled “Electromagnetic wave signal transmission method and system of measurement while drilling,” discloses an electromagnetic wave measurement while drilling method with formation as a transmission medium or with a drill string as a transmission conductor. Specifically, tested data are modulated onto an electromagnetic wave by a downhole instrument, emitted by an electromagnetic emitter from downhole, and then transmitted to the ground via various passages. Subsequently, a ground detector will detect electromagnetic signals modulated with the tested data, and a processing circuit will be used to demodulate the tested data contained in the electromagnetic signals. Document 4 (“Application of Acoustic Transmission Testing Technology in Oil Field,” Measurement & Control Technology, 2005, 24(11), pp 76-78) discloses use of acoustic waves or seismic waves for signal transmission via a drill shaft or formation. Specifically, an acoustic emission system mounted on a drill shaft modulates various tested data onto acoustic vibration signals, which will be transmitted to the ground along the drill shaft, and received by an acoustic receiving system arranged on the ground, followed by demodulation of the tested data from the acoustic vibration signals. Like electromagnetic transmission, in acoustic transmission, no slurry circulation is necessary, and therefore it is easy to achieve acoustic transmission at low costs. However, acoustic transmission is subject to the defects of fast attenuation, and susceptibility to environment, such as interferences of low intensity signals from the wellbore, and acoustic waves and electromagnetic waves from drilling devices, thus leading to difficult signal detection and low transmission speed thereof.
Therefore, there is an urgent need of a data transmission solution to the above problems, which can achieve fast transmission of the downhole MWD data to the ground at low costs.